The invention is characterized by the use of a small gas-fueled precombustion chamber to create an igniting flame for an injected charge of oil fuel. The system may be used in engines specifically designed for the relatively low firing pressures resulting from the new combustion arrangement, or the invention may be applied to existing gas engines where it becomes desirable to convert the engine for operation on primarily oil fuel.
The internal combustion engine art has long known that it was possible to convert a Diesel engine operating on fuel oil to a gas engine operating on natural gas by a fairly simple replacement of certain operating parts.
In the engines converted from a full Diesel cycle to a primarily gas-fueled cycle there was never any question about the strength of the engine components to withstand the conversion. A Diesel engine is characterized by a high compression ratio in the order of 12:1 to 18:1 and therefore by heavy frame and operating components to withstand the high peak pressures inherent in the Diesel cycle. On the other hand gas-fueled engines are characterized by a low compression ratio in the order of 8:1 or 9:1 and therefore by a relatively lighter frame and component construction because the peak pressures encountered in the gas engines are much lower than those encountered in Diesel engines.
Dual fuel engines have also been known for several decades in which a primarily gas-fueled engine had its charge ignited by injection of a small quantity of pilot oil. The compression ratio of such dual fuel engines was in the Diesel engine range to assure compression ignition of the pilot oil charge. Examples of a dual fuel engine may be found in Boyer U.S. Pat. No. 2,506,566 and examples of a converted Diesel-to-gas engine may be found in Friddell U.S. Pat. No. 3,187,728. It has also been proposed to operate a gas-fueled engine with an ignition source including an auxiliary combustion chamber in which a quantity of gas fuel was segregated and ignited. Such an engine is shown in Gehres U.S. Pat. No. 2,799,255. In this patent the main fuel charge is stated to be gases including methane through pentane of the hydrocarbon series. These fuels are by nature at the opposite ends of the combustion quality spectrum from oil fuel, with gas fuel having some measurable octane rating quality and a near zero cetane rating, and the liquid fuel having a reasonable cetane rating and a very poor octane rating.
These fuel differences present entirely different problems with regard to injection and combustion. The liquid fuel must be mechanically atomized and properly vaporized to permit the hydrocarbon elements to find the necessary oxygen. Due to the poor octane rating quality of the liquid fuel, it cannot be admitted to the combustion chamber too far in advance of the discharge from the precombustion chamber because of uncontrolled autoignition of a portion of the then fuel/air mixture. In the case of a conventional Diesel cycle with a good cetane fuel, the compression ratio is adequate to initiate combustion early in the fuel injection cycle. In the case of the present invention only sufficient heat is available to physically condition the fuel. Proper initiation of combustion by the precombustion chamber precludes undesirable autoignition by burning the injected fuel at the proper time, after it has experienced some chamber conditioning, but before it has accumulated in sufficient quantity to cause undesirable autoignition.
Recently the declining availability of natural gas has stimulated interest in the procurement of engines that could use liquid fuels to supplement or replace gaseous fuels. Nearly all of the natural gas pipeline compressor engines in use today operate on gaseous fuels and the desire to acquire new engines or to convert these existing engines to burn fuel oil without exceeding the stress levels imposed on engine components presents a difficult problem. As previously noted the structural strength of the typical gas engine is much lower than that of a comparable Diesel engine due to the much lower peak pressures involved in gas engine combustion. It is therefore apparent that a normal Diesel cycle (compression ignition) cannot be used in these existing engines so that while a Diesel engine can be converted to operate on gas, a gas engine cannot be converted to operate on oil fuel by known practices.